Balancer structure for three-cylinder engines

ABSTRACT

A balancer structure for a three-cylinder engine for eliminating the vibration in the engine especially the vibration caused by an inertia couple about an axis perpendicular to the crankshaft of the engine. A countershaft is rotated at the same speed as the crankshaft but in opposite direction. Two counterweights are secured to the crankshaft corresponding to the first and third cylinders of both ends for balancing of reciprocating masses and rotating masses. A counterweight is secured to the crankshaft for balancing of reciprocating masses. At least two balancers are secured to the countershaft at both ends thereof.

BACKGROUND OF THE INVENTION

The present invention relates to a balancer structure for three-cylinderengines, and more particularly to a device provided with a countershaftrotated at the same speed but in the opposite direction of thecrankshaft of the engine so as to balance the primary couple of inertiaforces of the crankshaft about an intermediate position in the axialdirection.

There are two inertia forces of reciprocating masses and rotatingmasses, causing vibrations in the engine. The inertia forces of rotatingmasses may be balanced by providing a counterweight on the crankshaft inthe opposite direction to a crank arm. The inertia forces ofreciprocating masses may be balanced by the counterweight by a half ofthe inertia forces and the remainder may be balanced by the countershaftwhich is rotated in the opposite direction from the crankshaft and atthe same speed.

However, in the three-cylinder engine, the inertia forces of the firstcylinder and the third cylinder act on the crankshaft symmetricallyabout an intermediate point corresponding to the second cylinder whichis disposed between the first and third cylinders. Thus, an inertiacouple acts about the intermediate point on the crankshaft. The coupleof inertia causes a considerable vibration in the engine. Even if theinertia forces of rotating masses and reciprocating masses are balancedand further if the couple of inertia about the X-axis is balanced, thecouple of inertia about an axis perpendicular to the crankshaft isinevitably generated. In order to balance such a couple of inertia,Japanese patent application laid open No. 55-6035 provides a balancerdevice of counterweights having a separated structure. Japanese patentpublication No. 54-2333 discloses a countershaft which generates acouple of inertia equal to the couple of inertia of the crankshaft butopposite to the direction thereof.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a balancer devicewhich can balance the couple of inertia about an axis perpendicular tothe crankshaft of an engine in addition to the inertia forces ofreciprocating masses and rotating masses.

According to the present invention there is provided a balancerstructure for a three-cylinder engine having three cylinders, acrankshaft and a countershaft rotated at the same speed as thecrankshaft but in opposite direction, comprising: counterweightssecurely mounted on the crankshaft corresponding to the first and thirdcylinders disposed in both ends of the engine; a counterweight beingdisposed on the crankshaft corresponding to the second cylinder disposedat an intermediate position; two balancers securely mounted on thecountershaft at both ends thereof.

The present invention will be more apparent from the followingdescription made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 to 6 are illustrations for explaining a principle of the presentinvention;

FIG. 7 shows an embodiment of the present invention;

FIGS. 8 and 9 are side views showing examples for automobile engines;and

FIGS. 10 to 14 illustrate other embodiments of the present invention,respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Explaining a balancing system for one cylinder with reference to FIG. 1,a crankshaft 1 has three crank arms 2 angularly equidistant by 120° withrespect to each other. A connecting rod 4 is connected to each crank arm2 by a crankpin 3 and to a piston 5. A counterweight 6 is secured to thecrankshaft 1 along a line extending from the crank arm and at theopposite side of the arm for the balancing of the entire of inertiaforces rotating masses and a half of inertia forces of reciprocatingmasses. A countershaft 7 is rotatably mounted in parallel with thecrankshaft 1 and is adapted to be rotated at the same speed as butopposite direction relative to the crankshaft. A balancer 8 is securedto the countershaft 7 for the balancing of the remainder of the inertiaforces of the reciprocating masses. The balancer 8 is so disposed thatturning angle θ of the balancer from the bottom on the Z-axis of thecountershaft 7 is equal to crank angle θ from the top dead center.

Presenting the inertial mass of reciprocating parts mp and, for theconvenience of explanation, the equivalent inertial mass at the crankpin3 of rotating parts mc, the mass of the counterweight 6 necessary foreliminating the vibration of the engine unit of FIG. 1 is mp/2+mc,because the mass of the counterweight 6 for balancing one half of thereciprocating inertial mass mp is mp/2 the mass for balancing the entirerotating mass mc is mc. On the other hand, the mass of the balancer 8necessary for balancing the remainder of the reciprocating mass is mp/2.Thus, the engine of FIG. 1 is balanced by the counterweight 6 and thebalancer 8 having the above described respective masses. Therefore, thetotal mass of the counterweight 6 a three-cylinder engine is3((mp/2)+mc) and the total mass of the balancer is (3/2) mp.

Explaining the balancing of the reciprocating inertial mass of thethree-cylinder engine with reference to FIG. 2, each of the firstcylinder to the third cylinder is designated by a numeral with suffix (ato c). In FIG. 2, the piston 5b of the second cylinder is at the topdead center, the piston 5a of the first cylinder is at 240° crank angleand the piston 5c of the third cylinder is at 120° crank angle.Vibration forces FP1 to FP3 of all cylinders at crank angle θ are asfollows, where r is the radius from the center of the crankshaft to thecrankpin and ω is the angular velocity of the crankshaft.

    FP1=mprω.sup.2 cos (θ+240°)

    FP2=mprω.sup.2 cos θ

    FP3=mprω.sup.2 cos (θ+120°)

The total inertia force is

    FP1+FP2+FP3=0

Therefore the vibration forces are balanced.

The couple of inertia of the crankshaft is expressed as

    FP1.S+FP2(S+L)+FP3(S+2L)

where S is a distance of a point P on the X-axis from the firstcylinder, L is a pitch between adjacent cylinders. The above formula issubstituted as follows. ##EQU1##

Thus, the couple of inertia about the Y-axis is produced in thecrankshaft by reciprocating masses in the Z-axis direction.

Explaining the half balancing of the inertia forces of the reciprocatingmasses by counterweights 6a, 6b and 6c with reference to FIG. 3, pistons5a-5c are in the same positions as FIG. 2 and each of counterweights 6a,6b and 6c is positioned at an angular position advanced 180° from thecorresponding crank arm 2a-2c.

Forces Frec1 to Frec3 caused by the mass of each counterweight in theZ-axis direction at a crank angle θ are as follows.

    Frec1=(mp/2)rω.sup.2 cos (θ+240°-180°)

    Frec2=(mp/2)rω.sup.2 cos (θ+180°)

    Frec3=(mp/2)rω.sup.2 cos (θ+120°+180°)

Therefore, inertia forces in the Z-axis direction are

    Frec1+Frec2+Frec3=0

Thus, the inertia forces are balanced.

The couple of inertia caused by the inertia forces in the Z-axis aboutthe Y-axis is expressed as ##EQU2##

Accordingly, couple of inertia about the Y-axis is also produced bymasses of the counterweights 6a-6c.

In addition, each of inertia forces of the counterweights 6a-6c has alsoa component in the Y-axis direction. The couple of inertia about theZ-axis is ##EQU3## Thus, the counterweights 6a-6c produce the couple ofinertia about the Y-axis and the couple of inertia about the Z-axis. Thecomposite couple of inertia is presented as ##EQU4##

It is to be noted that it is possible to remove a counterweight for thesecond cylinder and the counterweight is distributed to the first andthird cylinders. Explaining this with reference to FIG. 4, masses ofcounterweights 6a' and 6c' of first and third cylinders are (√3/2)(mp/2). The counterweight 6a' of the first cylinder is positioned inadvance 180°+30° from the crank arm 2a and the counterweight 6c' of thethird cylinder is positioned in advance 180°-30° from the crank arm 2c.That is counterweights 6a' and 6c' are 180° opposed and make a rightangle with the crank arm 2b.

The inertia forces of each cylinder by the counterweight in the Z-axisdirection at a crank angle θ is as follows. ##EQU5## The inertia forcesin the Z-axis direction are

    Frec1'+Frec3'=0

Thus, the inertia forces are balanced.

The couple of inertia by the Z-axis forces about the Y-axis is ##EQU6##This formula is the same as the formula (2a). The couple of inertiaabout the Z-axis is also the same as the formula (2b).

Thus, it will be understood that the inertia forces can be balanced byproviding counterweights for all cylinders or for the first and thirdcylinders and that the couple of inertia in both cases are the same.

The balancing of the couple of inertia about the Y-axis and Z-axis willbe explained hereinafter. The composite couple of inertia of formulas(1) and (3) is ##EQU7##

A system for balancing such a couple of inertia by the countershaft willbe described hereinafter with reference to FIG. 5. Balancers 8a, 8b and8c balance a half of the inertia forces of reciprocating masses, andhence each mass is mp/2. As shown in the figure, the balancer 8b for thesecond cylinder is at the bottom when the piston 5b of the secondcylinder is at the top dead center, the balancer 8a for the firstcylinder is at a position advancing 240°-180° from the top in thecounterclockwise direction, and the balancer 8c for the third cylinderis positioned advancing 120°+180°.

Therefore, the inertia forces by the balancers in the Z-axis directionat an angle θ are as follows.

    Frec1=(mp/2)rω.sup.2 cos (θ+240°-180°)

    Frec2=(mp/2)rω.sup.2 cos (θ+180°)

    Frec3=(mp/2)rω.sup.2 cos (θ+120°+180°)

Thus the inertia forces in the Z-axis direction are balanced.

The couple of inertia about the Y-axis by the inertia forces in theZ-axis direction is ##EQU8##

The inertia forces in the Y-axis direction are minus since thecountershaft rotates in the counterdirection. However, the inertiaforces are balanced.

The couple of inertia about the Z-axis by the forces in the Y-axisdirection is ##EQU9##

The composite couple of inertia of formulas (2a') and (2b') is ##EQU10##If the formula (4') and the formula (4) are combined, ##EQU11##

Thus, couples of inertias about an axis perpendicular to the crankshaftmay be balanced by balancers on the countershaft.

FIG. 6 shows an example in which the balancer for the second cylinder isomitted like FIG. 4. The mass of the balancer 8a' for the first cylinderand the mass of the balancer 8c' for the third cylinder is (mp/2) (√3/2)respectively. The balancer 8a' is advanced 30° from the position of FIG.5 and the balancer 8c' is retarded 30°. By these conditions, inertiaforces of rotating masses are balanced.

The balancing of the forces by rotating parts will be explainedhereinafter. The construction of the balancer device is the same as FIG.2. Forces Fc1, Fc2, Fc3 in the first to third cylinders at a crank angleθ are

    Fc1=mcrω.sup.2 cos (θ+240°)

    Fc2=mcrω.sup.2 cos θ

    Fc3=mcrω.sup.2 cos (θ+120°)

The couple of inertia about the Y-axis by the rotating masses is##EQU12## The couple of inertia about the Z-axis is ##EQU13## Thecomposite couple of inertia is ##EQU14##

The balancing system with the counterweights 6a, 6b, 6c for the coupleof inertia by the rotating masses are described hereinafter. Theconstruction of the system is the same as FIG. 3. Forces Frot1, Frot2and Frot3 by masses of counterweights 6a to 6c are

    Frot1=mcrω.sup.2 cos (θ+240°-180°)

    Frot2=mcrω.sup.2 cos (θ+180°)

    Frot3=mcrω.sup.2 cos (θ+120°+180°)

The couple of inertia about the Y-axis is ##EQU15## The couple ofinertia about the Z-axis is ##EQU16## The composite couple of inertia is##EQU17##

Thus, the composite couple of inertia of the formula (6) is alsobalanced by the composite couple of inertia of the formula (8).

The forces of rotating masses may also be balanced by separatingcounterweights into the first and third cylinders as described withrespect to FIG. 4. The mass of the counterweight is mc(√3/2) and thephase of the counterweight is advanced or retarded by 30°.

The present invention is based on the above described principle. Asdescribed above, inertia forces and couple of inertia are caused byreciprocating masses and rotating masses. In accordance with the presentinvention, a counterweight is mounted on the crankshaft for the secondcylinder only for the balancing of the reciprocating masses in additionto counterweights for the first and third cylinders for the balancing ofthe reciprocating masses and rotating masses.

Referring to FIG. 7, counterweights 6a-1, 6a-2, 6b-1, 6b-2, 6c-1, and6c-2 are provided for each cylinder opposite to the crankarm like FIG. 3for balancing the reciprocating masses. Further, for the first cylinder,two counterweights 6a'-1 and 6a'-2 are provided and counterweights 6c'-1and 6c'-2 are provided for the third cylinder for balancing rotatingmasses.

The countershaft 7 is provided with balancers 8a' and 8c' at positionscorresponding to bearings 9a and 9d of both ends except the secondcylinder. Since counterweights 6a'-1, 6a'-2, 6c'-1 and 6c'-2 for therotating masses are separated into two positions, each counterweight ismc(√3/2) and the phase of each counterweight is advanced or retarded by30° whereby they make right angles with the crank arm 2b (seedescription with respect to FIG. 4). If the pitch between the cylindersis L, the couple of inertia is balanced by the following conditions.##EQU18##

Therefore, presenting the composite mass of the counterweights 6a'-1 and6a'-2 "Mca'", and the composite mass of the counterweights 6c'-1 and6c'-2 "Mcc'", for the balancing of the inertia forces on the crankshaft1, it is necessary to keep Mca'=Mcc'.

Further, presenting the position of the composite center of gravity ofthe counterweights 6a'-1 and 6a'-2 in respect to the Y-axis "L+X'", andthe position of the composite center of the gravity of thecounterweights 6c'-1 and 6c'-2 "L+Y'", it is necessary to satisfy thefollowing equation, ##EQU19##

As to the counterweights 6a-1, 6a-2, 6b-1, 6b-2, 6c-1 and 6c-2 for themass of the reciprocating parts, it is necessary to keep Mca=Mcb=Mcc,where Mca, Mcb and Mcc are composite mass respectively. If the positionof the composite center of the gravity of counterweights 6a-1 and 6a-2for the composite center of the gravity of counterweights 6b-1 and 6b-2is L+X and the composite center of gravity of counterweights 6c-1 and6c-2 is L+Y,

    Mca(L+X)=Mcc(L+Y)

Thus, X=Y is necessary. For the couple of inertia in the Y-axisdirection, it is necessary to satisfy ##EQU20## This is changed to thefollowing general formula.

    Mca=Mcb=Mcc=(1/2)mpL/(L+X)                                 (9b)

Thus, the mass of the counterweight may be suitably determined independency on the composite center of the gravity. The mass decreaseswith the increase of X', Y', X and Y. If the center of gravity of thecounterweights 6a-1, 6a-2, 6a'-1, 6a'-2 of the first cylinder coincideswith that of the third cylinder and the center of gravity of thecounterweights 6b-1, 6b-2 of the second cylinder is positioned at thecenter of gravity, X'=Y'=X=Y=0. Therefore, the mass of the counterweightof the first and third cylinder is (√3/2)mc respectively. And the massof the counterweight of each cylinder is (1/2)mp. It will be noted thatalthough counterweights 6a-1 and 6a'-1 and others are separated by anangle of 30°, in practice these counterweights are integrally combined.

The masses of the balancers on the countershaft 7, as described above,correspond to the masses of the reciprocating parts of the engine andeach mass is (mp/2) (√3/2) and its phase is adjusted to 30°. The massesare so arranged as to produce the following couple of inertia, ##EQU21##

Therefore, considering the balancing of the inertia forces on thecountershaft, it is necessary to keep Mba'=Mbc', where Mba' is the massof the balancer 8a' and Mbc' is the mass of the balancer 8c'.

If the positions of the centers of gravities of the balancers 8a' and8c' are L+X" and L+Y", the following conditions are necessary: ##EQU22##As a result, ##EQU23## Thus, the mass Mba' and Mbc' also decrease withthe increase of X" and Y". Accordingly, the device may be made in asmall size, which resulting in decreasing of weight and space saving.

As is understood from the foregoing, the masses of the counterweights6a'-1, 6a'-2, and 6c'-1, 6c'-2 are provided to satisfy the equation (9a)and positioned to make a right angle with the crank arm 2b of the secondcylinder. Counterweights 6a-1, 6a-2, 6b-1, 6b-2, 6c-1 and 6c-2 offormula (9b) are opposite to a respective crankpin. On the other hand,the balancers 8a' and 8c' of the formula (10) are positioned,corresponding to the bearings 9a and 9d. The balancers 8a' and 8c' areso disposed that when the piston of the second cylinder is at the topdead center, the balancer 8a' is at the same angular position as thecounterweights 6c'-1 and 6c'-2 and the balancer 8c' is at the sameangular position as the counterweights 6a'-1 and 6a'-2. Thus, theprimary inertia forces, primary couple of inertia of the reciprocatingand rotating parts of a three-cylinder engine and the primary couple ofinertia of the crankshaft about axes perpendicular to the crankshaft arebalanced by the counterweights on the crankshaft and balancers on thecountershaft.

Since the balancers 8a' and 8c' are disposed corresponding to thebearings at both ends so as not to engage with the counterweights, thecountershaft may be disposed adjacent to the crankshaft and the rigidityof the engine may be increased. In addition, since the counterweightsfor the reciprocating masses are provided for the first and thirdcylinders far from the second cylinder, the masses of the balancers canbe reduced. Namely, the engine may be made of a small size compared withan engine provided with counterweights at every cylinders.

FIG. 8 shows an example in which an engine 10 is transversely mounted onan automobile at a rear portion thereof for driving the rear wheels. Anair cleaner 11, carburetor 12 and induction pipe 13 are horizontallydisposed and connected with each other. Further, a compressor 14 for anair conditioner and ACG 15 are also attached. In accordance with thepresent invention, the countershaft 7 can be disposed adjacent to thecrankshaft 1 without interfering with upper devices such as thecarburetor 12 as well as without sinking into the oil in the oil pan.

FIG. 9 shows another example in which the engine 10 is transverselymounted on an automobile at a front portion for driving the frontwheels. An exhaust pipe 16 and a catalytic converter 17 for an emissioncontrol system are disposed in a front side of the engine. In thisexample, the countershaft 7 can also be disposed adjacent to thecrankshaft 1 without interfering with such the equipments.

Referring to FIG. 10 showing the second embodiment of the presentinvention, each of balancers 8a' and 8c' of this embodiment is formed asa part of a journal for bearing the countershaft 7. The balancer 8a' iseccentrically formed in a cylindrical journal 20 which is coaxial andintegral with the countershaft 7. The journal 20 is supported by ajournal bearing 21 in a frame 22 which supports the crankshaft 1 by thebearing 9a. The balancer 8c' is formed in the same manner as thebalancer 8a' and supported in a frame 24. Other parts are the same asthe first embodiment of FIG. 7.

In accordance with this embodiment, extra portions for bearing thecountershaft are not provided, whereby the device may be assembled intoa small size.

In the third embodiment of FIG. 11, balancer 8a' is divided intobalancers 8a'-1 and 8a'-2 and balancer 8c' is divided into 8c'-1 and8c'-2. In this embodiment, composite mass Mba' of the balancers 8a'-1and 8a'-2 and composite mass Mbc' of the balancers 8c'-1 and 8c'-2 mustbe equal. Each of balancers 8a'-1 and 8c'-2 at both ends is formed asjournal 20 as the second embodiment and supported by the journalbearings 21.

In the fourth embodiment of FIG. 12, the balancer comprises threebalancers 8a, 8b and 8c according to the principle of FIG. 5. Also inthis embodiment, mass Mba, Mbb and Mbc of balancers 8a, 8b and 8c mustbe equal each other.

FIG. 13 shows the fifth embodiment of the present invention. In thisembodiment, the balancer comprises three balancers 8a, 8b and 8c likethe fourth embodiment. Each of balancers 8a and 8c at both ends isformed as journal 20 and the same parts as FIG. 11 are identified by thesame numerals.

In the sixth embodiment shown in FIG. 14, the central balancer of thefifth embodiment of FIG. 13 is divided into balancers 8b-1 and 8b-2. Thecenter of gravity of the balancers 8b-1 and 8b-2 is positioned at acenter of the second cylinder. The balancer 8b-1 corresponds to abearing 9b for the crankshaft 1 and the balancer 8b-2 corresponds to abearing 9c. Thus, the space in the engine may be effectively used.

What is claimed is:
 1. A balancer structure for a three-cylinder in-lineengine having three cylinders, a first and third cylinder, and a secondcylinder between said first and third cylinders, a crankshaft havingcrank arms disposed at angles of 120° with respect to each other andoperatively connected to said cylinders, respectively, consisting of:asingle countershaft adjacent and parallel to and rotated at the samespeed as the crankshaft but in the opposite direction; means comprisingfirst counterweights securely mounted on said crankshaft only atpositions thereof corresponding to the first and third cylinders forbalancing a part of inertia force of reciprocating masses and the entireinertia force of rotating masses; means comprising at least one secondcounterweight securely mounted on said crankshaft substantially oppositeto the crank arm corresponding to the second cylinder for balancinganother part of the inertia force of the reciprocating masses; at leasttwo balancers securely mounted on said countershaft at both ends thereoffor the balancing of the remainder of the inertia force of thereciprocating masses and a couple of inertia of the crankshaft about anaxis perpendicular to the crankshaft.
 2. The balancer structure for athree-cylinder engine according to claim 1 wherein said balancers aredisposed in positions corresponding to bearings for both ends of saidcrankshaft.
 3. The balancer structure according to claim 2, whereinsaidbalancers are positioned on said countershaft axially offset withrespect to said first counterweights so as not to engage with the firstcounterweights while crossing axially offset during rotation, saidbearings are disposed such that said first counterweights aretherebetween.
 4. The balancer structure according to claim 3,whereinsaid countershaft is closely adjacent to said crankshaft.
 5. Thebalancer structure according to claim 1, whereinsaid firstcounterweights include third counterweights for the first and thirdcylinders for the balancing of the rotating masses as well as fourthcounterweights for the balancing of the first-mentioned part of thereciprocating masses, said third counterweights are separated from acorresponding of said fourth counterweights by an angle of plus andminus 30°, respectively.
 6. The balancer structure according to claim 5,whereinsaid fourth counterweights and said third counterweights are atleast partially integrally combined.
 7. The balancer structure accordingto claim 6, whereinsaid third counterweights form a right angle withrespect to the crank arm for the second cylinder, and said fourthcounterweights for said first and third cylinders, respectively, arepositioned 180° with respect to a first and third of said crank arms,respectively, corresponding to said first and third cylinders.
 8. Thebalancer structure according to claim 1, whereinsaid firstcounterweights include third counterweights for the first and thirdcylinders for the balancing of the rotating masses, respective of saidthird counterweights for the balancing of the rotating masses isrespectively advanced and retarded by 30° from a position opposite afirst and third of said crank arms, respectively, corresponding to saidfirst and third cylinders, such that said third counterweights extendperpendicular to said crank arm corresponding to said second cylinder.9. The balancer structure according to claim 8, whereinthe cylinders arealigned spaced by equal distance therebetween.
 10. The balancerstructure according to claim 8, whereinsaid third counterweights areeach formed in pairs and have composite masses which are equal anddisposed at equal distances from a central axis perpendicular to saidcrankshaft passing through said crank arm corresponding to said secondcylinder.
 11. The balancer structure according to claim 1, whereinsaidfirst counterweights include third counterweights for the first andthird cylinders for the balancing of the rotating masses, the masses ofsaid third counterweights are equal.
 12. The balancer structureaccording to claim 11, whereineach of said balancers at respective endsof said countershaft are disposed at the same angular position as saidthird counterweights at remote ends of said crankshaft respectively whensaid second cylinder is in a top dead center condition.
 13. The balancerstructure according to claim 1, whereinsaid balancers are positioned onsaid countershaft axially offset with respect to said firstcounterweights so as not to engage with the first counterweights whilecrossing axially offset during rotation.
 14. The balancer structureaccording to claim 13, whereinsaid countershaft is closely adjacent tosaid crankshaft.
 15. The balancer structure for a three-cylinder engineaccording to claim 1 wherein each counterweight comprises a pair ofweights.
 16. The balancer structure according to claim 1, whereinthecylinders are aligned spaced by equal distance therebetween.